1. Field of the Invention
The invention relates generally to magnetic recording devices. The invention relates more specifically to an apparatus and method for positioning a magnetic head relative to the tracks of a multi-track tape.
2a. Cross Reference to Related Applications
The following copending U.S. patent application(s) is/are assigned to the assignee of the present application, is/are related to the present application and its/their disclosures is/are incorporated herein by reference:
(A) Ser. No. 07/794,999 by Nayak et al. and entitled GEAR DRIVE CARRIAGE AND STEPPER ADJUSTMENT SYSTEM;
(B) Ser. No. 07/926,743 by Nayak et al. and entitled MECHANISMS FOR A CLOSED LOOP HEAD POSITIONER FOR STREAMING TAPE DRIVES.
2b. Cross Reference to Related Patents
The following U.S. patent(s) is/are assigned to the assignee of the present application, is/are related to the present application and its/their disclosures is/are incorporated herein by reference:
(A) U.S. Pat. No. 5,191,492 issued Mar. 2, 1993, to Nayak et al. and entitled MECHANISMS FOR A CLOSED LOOP HEAD POSITIONER FOR STREAMING TAPE DRIVES.
3. Description of the Related Art
High-density recording of information on multiple tracks of a magnetic tape is well known. Parallel tracks are defined to extend along a substantially longitudinal direction of an elongated magnetic tape. A magnetic head is moved in a transverse, lateral direction across the tape surface to bring a read and/or write gap of the head into proximity with a desired track prior to recording or playback. During a recording or playback session, the head is expected to remain on track while the tape moves in the longitudinal direction, past the read/write gap.
Accurate positioning of the head's read/write gap to a desired track is important. Two basic kinds of head positioners are used for providing head to track alignment: open-loop and closed-loop.
Open-loop positioners are typically employed for one-time placement of a head relative to a track and are commonly found in tape systems having relatively low track densities. The magnetic head rides on a lead screw. A stepper motor rotates the lead screw and the lead-screw converts the rotational motion of the motor into linear movement of the head. The stepper motor is advanced a fixed number of degrees to shift the head from one track to the next prior to reading or writing. The head remains in a fixed position during the read/write session. No provision is made for correcting alignment error while the tape moves and information is being read from or recorded onto the track.
Closed-loop positioners are typically used for multi-track tapes having relatively high track densities and high recording/readback rates. The tape has a tendency to disadvantageously wander in the lateral direction as it advances in the desired longitudinal direction. This creates an undesirable track-to-head misalignment. If tracks are spaced very close to one another, the lateral wander can be sufficient to produce an off-track condition. To overcome this problem, servo signals are pre-recorded onto the tape to mark the position of each track. A closed-loop servo system moves the head laterally while searching for the servo signals of a desired track and thereby brings the head into fine alignment with the desired track. Feed-back is used to continuously maintain alignment with the servo signals during the reading or writing of information from/onto the track.
A combination of an open-loop coarse positioner and closed-loop fine positioner has been proposed for positioning a magnetic head relative to a high-density multitrack tape. The above cited U.S. Pat. No. 5,191,492 discloses such a combination. A voice coil is attached to the magnetic head for providing closed-loop fine positioning while the head and voice-coil assembly rides on a lead screw that is driven by a stepper-motor to provide coarse positioning.
Such a combination of an open-loop, coarse positioner and a closed-loop, fine positioner suffers from the following drawbacks. Numerous parts are needed both for individually constructing the coarse and fine positioning subsystems and for connecting the two subsystems together. The cost of manufacture for such a combination is large due to the additive cost of the individual parts and due to the work involved in combining so many parts. The combined size of the coarse and fine positioning subsystems tends to be disadvantageously large. Integration of the open-loop and closed-loop servo electronics is complex and raises stability problems. Track switching time and/or track switching power consumption tends to be large due to the combined mass of the coarse and fine positioning subsystems.